Balanced transmission line oscillator



OIIIL 284, 1947. E, o, w|| 0UG|-|BY y 2,429,656

BALANCED. TRANSMISSION LINE OSCILLATOR Filed Feb. 27, 1942 ,5; @ww @Mfrom/Ey y elements.

Patented Oct. 28, 1947 BALANCED .TRANSMlSS/,IQN i oscmLA'ron- I Eric-Osbome Willoughby, v Londonf. Englandif. as.. `signolgoto, InternationalStandard .Elec/triatlon peraiicnewcllt N Yr;

y aclantis. (cresci-sc),

1 The present inventionfrelatesmte short- :ware@adentrashrt-Waveescllaters andrartcularly unbalancedbscillators, i.efioscillator usfizvigfa single Valve lor-two or more valves.inparalljjel-,fand according. thereto, a trimming Acondenser tied; to

the Plaie :lineisarrfvided i0 :balance fthe endend platetransmission-linesi0.-@Meede- 'The detention will l iba Ebene-riunderstood :from thefffollowingdescriptions-staken orliunctien WithV theawmneeyine drawiriesf.;Which.;i1lus trate. ultraeshcrt .WaY-eSQiIlatQrS:It: Willibe undersiflod; howeverithat. the inventif). is equally:applicable toshQrt-iwavetoscillators :with larger valves,

In the drawings,

Figs. 1a, 1b and 1c illustratefknewn,:types of ultra-short waveoscillatorsutilising lengths? of transmissiondine together- .with theinterelec .ode capacity of a thermionicgyalyeffas theI tank ci.:` itThe, earth f is. nermallyj thctscreening case of the oscillator. Y

Figure 2 :shows ,schematicallya tank` cir-cuit for an. ultrashortWaVeoscillator-embodying the inventiorriwfandl Figure 3 rshowsa.sligl-itly- ,modied arrange: ment.`

In, Fig. 1a, the :oscillator Valve ,..ontubet Twine4 cludes aIcathode.-.C; grid-. Gn and man, anode eA.: One-quarter. Hvrfaifelen'gthItransmission line iconsductors` GL and are `connected..arespectiytelyrWiththe. grid. and anode.l of.yalved'iatK theright hand end of lineAL,GL,= thelefthandiendloffithe line being 4effectively shortcircuitedithrough the condenser plates E. The lineAL; GL incombmaf,

- tion With the. inherent capacity -.Cyc;.betweentlie.

grid G and cathode.C'randrxwithvthe inherente capacity I Cpc..bet'\ife'en` the .anode .A rand.. cathode; C, servesv as a tankcircuit.. A positive hignitena sioni/voltage` source connected-throughthe radio frequency chokecoil BFC with the left end of line AL forsupplying anode current to the valve reconnection :iseprevded firent@grclmd through grid leak resistance Myvithhelefk endof line GL for thegrid G. A line. CI-,LO adjustable length, connects from ground with thecathode C. The screeningcanaor shield Kaprow vides an earth, orgroundconnectionV torathe' circuits; ofthe oscillator.

In Fig. la, valve, T is connected at the-right hand end of line.AL. GL,ythe lett. end of the line being open, and the connections for coil RFCand leak M being made at thecenterof the liner..

In Fig.` lcT the.valveffwis;.connected-atllthe center of line AL, GL,the connections for coil RFC and leak M being made near the right end ofthe line.

Referringfto 1a,- 1b 'and flc, in-'suclfrv cire` cuits .asillustrated-'the grid cathodecapacity Cgcis .inherentlylarger than theplate cathode capacity C'pc,. and there is an unbalanced current toearth-through-the cathode return. lead :CL which isccmmonly tuned to theoscillationy by varyingitheiengthfof transmission line attached te thefilament cathode terminals of the valveand returned to the screening canor shield K-o'f the' oscillator.

ItI now appearsrthat lsuch-tuning of .thecathode' line can have noappreciable eiiect. on balancing the oscillator tank circuiti-and incarrying out this.v invention a `low .impedance is provided-in thecathode to earth' pathlisuch for= instance-.as may often be obtained bydirectly short circuiting the cathode to oscillator. screening canK):.and

the circuit is tuned by means of a small trimming condenserfbeiweenplatel lead and earth or screen lcamthereby; providingaJ parallelreactance between plate andmscrevenr can or earth which Vre-V -actancecan begmade to balance. the plate and grid `transrnissionv lines of thevoscillatortank circuit about earth or the screening can.

' Referring to yFigure 2,; the; tank circuit may be' modiiied to any ofthe transmission line arrange-Y4 ments known to the art and typicd byFigures 1a., 1b and 1c.

The anode of thevalve.4 is indicated at A, contml.. aridi:v at.:.G,aa1idthe. cathode (indirectly heatcdfoverffthe@circuit frQm; the 10Wvvoltage; source LT to ground) at C. Anode andlgridlcads; are shownrespectively at ALvand GLaslengths of concentric transmission line andthecathode lead as CL, itsy inducta'nce being represented by L.Outputfrom-.the valve. is obtained by means of a coaxialzline LO. Thesmall trimming condenser canfis indicatedat ,Cba This condenser may be.

connected between. lthe pla-te lead ALI-and'. cathode directly iii-:itispractically more convenient.- The outer'-t-:ondu'ctorsy of the. plateand grid transmis--` sion.- =linesy @are earthed for vhigh frequency'.cur-- rentsthrcughfzpl-'ates EV` forming vcondensers-.with thex'screen-ingA caln-Kf, a; sheet. .of .mica N` being inserted desired,between E' andi-Kt.

Thisrbalancingg.adjustment of condenserv `Cb gives'"strongestfoscillationand maximum output and in practice .thebalancingpoi-rit.is-:shownv most markedly y.bythegrid.l current. which reaches amaximum value corresponding.. tok maximum plate and..gridf.vol'tageswings..the grid vleak being increased,iiinecessary, to :maintain .thegrid dish ssination.- at. a reasonable xvalue...

It is well to note that the essential adjustment is an increase ofplate-cathode capacity by an amount corresponding to Cgc-Cpc, theadjustment being affected by the inductance of the leads to theelectrodes inside the Valve, andin practice the adjustment amounts to abalance f plate-to-cathode and grid-to-cathode reactances of the valveand tank circuit in the oscillator.

Since the grid line GL is already damped by the grid leak M and sincethe trimming condenser Cb adjusts reactance of the plate line AL for anunbalanced output, it is advisable to tap the output lead LO directly onthe plate line, for'if the transmission line is correctly matched at theload end, due to the high ratio of volt-amperes to watts in the shortwave circuits, the reactance adjust-l `ment of the trimming condenser Cbwill necessarily be small. It should be noted that although push-pullarrangements of valves are convenient for oscillators, they result inrelatively high capacity across the transmission lines and limit thefrequency that can be obtained, while use of inverted valves, that is,of the grounded grid type, in pushpull although reducing this capacityeffect inherently reduces the ratio of alternating anode voltage toAdirect current anode voltage and their eiective eiiiciency, hence theabove balancing circuit for single valve oscillation often is the mostsatisfactory solution of the problem of obtaining a desired output witha given value at a relatively high frequency. v

Theory y Referring to Fig. 2, the unbalance current through AL to earthis: mgl/ l(Xcpc--X-cgc) (Kw-Xen) (XL-Kapu) (XL-Xeye) Since Cgc isinherently greater than Cpe from the construction of the valve, thebalancing method must be resorted to, to balance the transmission linetank circuits to earth. It isv important to notice, however, that sinceL the inductance of the cathode lead carries the unbalance current, solong as its reactance is reasonably small'such as often can be obtainedmerely by shorting the cathode to the screening case of the oscillator,it will have little effect on the amplitude of the grid voltage and theonly tuning necessary becomes the balancing adjustment provided thetransmission lines are correct to length, otherwise a small trimmingcondenser will be required across the lines for tuning the valve.

The balancing and trimming condensers may be combined as two seriescondensers in an arrangement as indicated in Fig. 3 in which C1 and Czare two condensers in series across the plate 1 Cz namely,

is the trimming or tuning adjustment.

Magnification factor (n) of valves The type of oscillator shown in Figs.1a, 1b

Y. and 1c is eminently suitable for low a tubes, and

the grid voltage swing obtained by the arrangement according to thisinvention is higher than is desirable for a good output with high valuesof ,u., the resistance of the grid leak having to be increased to anextent that makes the operating angle of the valve very small.

In this case a capacitative reactance placed in the cathode and filamentleads (i. e. in series with the inductance L) is necessary to reduce thedrive to the grids to a satisfactory value, the unbalance current toground due to Cg being larger than Cpe will then result in thegrid-cathode voltage swing being reduced and the drive brought to asatisfactory value without too high a value of grid leak, resulting intoo small an operating angle. A length of transmission line or a smallcondenser in the cathode lead will then be satisfactory. This, however,does not do away with the need of balancing and the balancing condenserCb is still necessary.

What is claimed is:

1. An oscillator for short and ultra-short waves including a valvehaving grid and plate electrodes and a cathode, a grounding impedancefor saidA cathode, saidimpedance having someinductive reactance inherenttherein at the frequencies involved, the grid-cathode internal capacityof the valve being greater than the plate-cathode internal capacity,transmission line circuits coupled to said electrodes, and a trimmingcondenser, having a capacity corresponding to the diierence between saidcapacities, connected externally of the valve between the plate line andground, arranged to balance said transmission lines with respect to thecathode, and to tune the out-of-balance current to ground through saidinherent cathodegrounding impedance.

2. The oscillator of claim 1 wherein the transmission line circuitscoupled to said electrodes comprise anode and grid leads terminating ingrounded condensers and wherein the output of said Oscillator is takenat a point on the anode lead in advance of said grounded condensers.

ERIC OSBORNE WILLOUGHBY.

REFERENCES CITED The vfollowing references are of record in the illeofthis patent:

UNITED STATES PATENTS Number Name Date 1,995,175 Gill Mar. 19, 19352,068,990 Kolster Jan. 26, 1907 2,243,574 Nickle May 27, 1941 FOREIGNPATENTS Number Country Date 659.531 Germany May 5, 1938

